The present invention relates generally to the field of optical imaging. More specifically, the invention relates to xerographic printers and, more particularly, to xerographic printers that incorporate a Vertical Cavity Surface Emitting Laser (VCSEL) array whose output is corrected by beam intensity variations through the use of process control electronics.
Polygon Raster Output Scanner (ROS) printers typically consist of a modulating laser light source, a polygon scanning beam deflector, an optical system of lenses and mirrors, a xerographic marking engine and the electronics to control printer operation. The ROS is positioned in an optical scan system to write an image on a moving photoreceptor surface. In the ROS system, a modulated beam is directed onto the facets of a rotating polygon mirror, which then sweeps the reflected beam across the photoreceptor surface. Each sweep exposes a raster line to a linear segment of a video signal image.
However, the use of a rotating polygon mirror presents several inherent problems. Bow and wobble of the beam scanning across the photoreceptor surface result from imperfections in the mirror or even slight misangling of the mirror or from the instability of the rotation of the polygon mirror. These problems typically require complex, precise and expensive optical elements between the light source and the rotating polygon mirror and between the rotating polygon mirror and the photoreceptor surface. Additionally, optically complex elements are also needed to compensate for refractive index dispersion that causes changes in the focal length of the imaging optics of the ROS.
The modulating laser light source may consist of a Vertical Cavity Surface Emitting Laser (VCSEL) array. The VCSEL array may be either a one or two-dimensional array of individual laser sources. Each individual laser source in the VCSEL array has a corresponding drive circuit which may be used to generate a beam to expose a corresponding area on a moving photoreceptor in response to video data information applied to the drive circuits of the VCSEL array. The photoreceptor is advanced in the process direction to provide a desired image by the formation of sequential scan lines generated by the beam to beam exposure delivered from the VCSEL array.
Current beam to beam uniformity correction in multi-beam ROS systems multiplexes one photo detector and one loop-back system among all the beams. This is accomplished by sequentially selecting each beam and comparing the output of the photo detector for that beam with a “reference” to decide whether to increase or decrease the power intensity in the selected beam. This sequential detection and power adjustment process takes a few micro-seconds per beam. In a high performance multi-beam ROS system the beam to beam uniformity correction has to be fine-tuned for each beam per scan line. In a VCSEL ROS system incorporating a two-dimensional array for producing a plurality of beams, the sequential beam to beam uniformity correction may take up to a few hundred micro-seconds. This uniformity correction scheme presents an amount of delay time for each line to be printed such that it renders the sequential multiplexing of all the beams unusable for high speed/high performance platforms.